Figure 3.

The structures of the actin filament, myosin thick filament, and their packing in the sarcomere, drawn to scale. (a) Actin is a single-start left-handed helix, more commonly described as two protofilaments with a right-handed twist with a repeat of 72 nm or pseudo-repeat of 36 nm. The myosin filament, approximately three times thicker, consists of myosin heads arranged in a three-start right-handed helix. The end-on view is shown on the right. The periodicities of the actin filament and myosin heads in the thick filament are not related. (b) Schematic representation of a cross section of a cardiac sarcomere showing a hexagonal packing of myosin thick filaments with one actin thin filament at each trigonal position (64). The packing is quite dense with heads very near the actin even in the resting state. To see this figure in color, go online.